The insulin-like growth factor (IGF) axis involves complex interactions among a number of different signaling factors (IGF-1, IGF-2 and IGF-3), their various cognate binding proteins (IGF-BPs), as well as both IGF, insulin, and IGF-insulin hybrid receptor proteins (IGF-R, INS-R, and IGN-R, respectively). IGFs are growth stimulatory peptides, structurally similar to insulin, that participate in the regulation of mitogenesis, cellular differentiation, and apoptosis. Normally, IGF-1 is produced predominately by the liver and largely functions as an endocrine hormone. Alterations in the IGF-1 signaling pathways have been described in multiple tumors including osteosarcomas, breast, bladder, gynecological, gastrointestinal, prostate and lung cancers. Animal and human studies have shown that in such cancers IGF-1 also functions as a paracrine and autocrine hormone, being produced by the tumor cells and interacting with IGF-R, which is frequently overexpressed by the tumor cells as well. [Arnaldez and Helman, Hematol. Oncol. Clin. North Am., 26:527 (2012)]. Numerous studies have established a relationship between high serum levels of IGF-1 and increased cancer incidence and mortality. Thus, the IGF axis provides new opportunities to develop effective cancer therapeutics. Several therapeutic approaches to exploiting the IGF axis have been explored, including various strategies for blocking IGF-R function as well as increasing the availability of IGF-BPs. [Heidegger, et al., Cancer Biology and Therapy 11:701 (2011)].
IGF-R was first identified as a promising therapeutic target over 20 years ago when Arteaga and Osbourn reported that antibodies against IGF-R inhibited growth of breast cancer cells in vitro [Arteaga and Osborne, Cancer Res 49:6237 (1989)]. Since then, as many as 30 different agents targeting IGF-R have been developed and over 60 clinical trials evaluating anti-IGF-R therapies have been reported [reviewed in Heidegger et al., Cancer Biology and Therapy 11:701 (2011)]. The majority of agents targeting IGF-R are monoclonal antibodies (mAbs), which exhibited good safety profiles in early, Phase I and II testing. However, more recent Phase III trials, in combination with different drug treatments indicated that there may be some problems with this approach. In March 2010, Phase III trials of one of the more successful anti-IGF-R mAbs, figitumumab, used in combination with either paclitaxel or erlotinib (an epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI)) were halted because early results indicated that the combination of drugs was not significantly effective over either drug alone, as well as safety concerns that the combination drug cohort experienced an elevated level of adverse effects. Interestingly, high levels of free IGF-1 appeared to be a marker for resistance to figitumumab therapy [Gualberto et al (2010b) Br J Cancer 104: 68 (2010)]. Significantly, one of the side effects seen in the figitumumab treated group was hyperglycemia, suggesting that that inhibition of IGF-R effects cross-talk with INS-R. Despite this setback, figitumumab and related anti-IGF-R mAbs remain among the best therapeutic candidates available and methods to increase treatment efficacy and prevent or reduce side effects would be invaluable for deploying this drug class in routine clinical use.
One alternative to anti-IGF-R mAb therapy targets the signal transduction tyrosine kinase activity of IGF-R (IGFR-TKI). The biochemical strategy is similar to EGFR-TKI drugs such as gefitinib, erlotinib and others, which have already been approved for treatment of lung, hepatocellular and renal cancers. Essentially, IGFR-TKIs compete for the ATP binding site of IGF-R and block the transition of the receptor to the phosphorylated active conformation. Initial clinical results report similar side effects as observed with IGF-R mAbs, especially with respect to hyperglycemia. Thus, IGFR-TKI treatment seems to suffer the same problems with cross-talk between the IGF-R and INS-R systems. In fact, the effect may be due to more than just cross-talk since the ATP binding domains of IGF-R, INS-R and IGN-R are virtually identical and IGFR-TKI drugs likely bind each of the receptor species with equivalent or nearly equivalent affinity.
A third approach to targeting IGF-R for cancer therapy involves antisense oligonucleotides (IGFR-ASO) that selectively target and destroy IGF-R transcripts prior to translation. Preclinical studies have developed a number of promising candidates, including at least one IGFR-ASO capable of suppressing growth of a paclitaxel resistant prostate tumor model. The specificity of the IGFR-ASO strategy holds great promise for avoiding the cross talk issues observed with IGF-R mAb and IGFR-TKI therapies, in particular the tendency to provoke hyperglycemia. However, the biology of the system is complex and it isn't certain that IGFR-ASOs will prove clinically useful. A pilot clinical study indicated that the IGFR-ASO was well tolerated but this approach suffers from poor half-life and delivery problems. No oral delivery route is available and routine clinical use of such compounds will require overcoming this limitation.
One other aspect of the IGF axis has been explored for its potential as a cancer therapeutic. In this approach, the availability of IGF-BPs is increased to reduce available free IGF capable of activating IGF-R. IGF-BPs are quite selective for their specific cognate IGF and preclinical work has shown that these proteins exhibit pro-apoptotic, anti-proliferative and anti-angiogenic properties. Unfortunately, there are no clinical studies of this approach and not all in vitro results have been replicated in animal models. Though promising, much work remains to be done to develop this strategy into a useful therapy.
Other than the early stage work with IGF-BPs, no method for directly limiting the availability of IGF-1 to limit activation of IGF-R is known. A strategy for reducing the level of IGF-1 available to activate IGF-R, without provoking cross talk between IGF-R and INS-R involving a safe, orally delivered drug would be of great value to the field.
Tamoxifen is a triphenylalkylene derivative that binds to the estrogen receptor (ER). It has both estrogenic and antiestrogenic actions, depending on the target tissue. It is strongly antiestrogenic to mammary epithelial cells, hence its use in both the prevention and treatment of breast cancer. Tamoxifen was originally screened in a program oriented to discovering new contraceptive agents. Although it was not a useful drug for control of fertility, tamoxifen was eventually discovered to be useful for clinical treatment of breast cancer. The therapeutic mechanisms of tamoxifen are complex, the primary effect of tamoxifen is exerted via estrogen receptors, but the drug may also modulate IGF-1 levels as well. However, at least one study reported no change to circulating IGF-1 levels after tamoxifen treatment [Campbell, et al., J. Clin. Pathol: Mol Pathol. 54:307 (2001)]. In vitro studies suggest that tamoxifen may also disrupt IGF-1 autocrine loops in at least some cancer cell types, but has no effect in others [Howe, et al., Cancer Res. 56:4049 (1960].
Tamoxifen is a pro-drug requiring metabolic activation by hepatic cytochrome P450 enzymes. In particular, CYP2D6 is instrumental in converting the pharmaceutically inactive tamoxifen and its most predominate metabolite, N-desmethyltamoxifen to endoxifen (4-hydroxy-N-desmethyl-tamoxifen), the pharmaceutically active form of the drug, which has much higher affinity to the ER than either of its precursors. CYP3A4 also plays a key role in activating tamoxifen or 4-hydroxy-tamoxifen to the N-desmethyl form. Extensive pharmacogenomic analyses of tamoxifen metabolism show that certain human alleles of CYP2D6 are incapable of activating tamoxifen to endoxifen and thus patients with these alleles receive no benefit from treatment with tamoxifen.
Another structurally similar triphenylalkylene derivative with both estrogenic and antiestrogenic activities is clomiphene. Clomiphene blocks normal estrogen feedback on the hypothalamus and subsequent negative feedback on the pituitary. This leads to increases in luteinizing hormone and follicle stimulating hormone. In men, increased levels of these gonadotropins results in the production of higher testosterone levels from the Leydig cells of the testes. In women, these increased levels of gonadotropins results in ovulation. Clomiphene citrate has been used to treat female infertility for many years with a relatively low level of serious side effects.
Ernst et al., J. Pharmaceut. Sci. 65:148 (1976), have shown that clomiphene is a mixture of two geometric isomers which are referred to as cis,-Z-, clomiphene (cis-clomiphene, or zuclomiphene) and trans-,E-, clomiphene, (trans-clomiphene or enclomiphene). Ernst et al. also noted that (the trans-isomer) is antiestrogenic, while the cis-isomer is the more potent and more estrogenic form, but has also been reported to have anti-estrogenic activity [Ibid.]. Recently, the isolated trans-isomer of clomiphene has been developed to treat, inter alia, secondary hypogonadism in men and is currently in Phase III trials as Androxal®.
Like tamoxifen, clomiphene is metabolized to the 4-hydroxy and N-dealkyl forms by the liver enzymes CYP2D6 and CYP3A4, respectively [Ghobadi, et al., Drug Metab. Pharmacokinet 23:101 (2008) and Murdter et al., Hum. Mol. Genet. 21:1145 (2012)]. Murdter has also shown that 4-hydroxy-trans-clomiphene ((E)-40H-clomiphene) and N-desethyl-4-hydroxy-trans-clomiphene ((E)-DE-4-OH-clomiphene) are strong ligands for the human estrogen receptor [Ibid.]. The fact that both tamoxifen and clomiphene are both activated by the same liver enzymes suggests that other triphenylalkylene derivatives may also produce active pharmaceutical compounds when metabolized by these enzymes. In addition, the fact that both compounds act as antiestrogens implies that they may share similar anticancer activities as well as other useful pharmaceutical properties.
In the course of drug development, the inventors observed that treatment of men with isolated trans-clomiphene was accompanied by a clear and significant reduction in IGF-1. Thus, specific derivatives of clomiphene and related triphenylalkylene derivatives represent a class of new cancer therapeutics targeted to the IGF axis. The pharmaceutical properties of these compounds suggest that they represent attractive less toxic alternatives to tamoxifen and other IGF axis targeted therapies.